Ripple reducing devices for reducing ripple of a fluid flowing in a hydraulic line are known. For example, an apparatus as disclosed in Japanese laid-open Patent Publication No. 60-40720 is adapted to reduce suction noise or sound that arises when secondary air is introduced into an engine of a motor vehicle. This apparatus includes a silencer or noise eliminator provided on the upstream side of a check valve of a secondary air passage communicating with a secondary air supply port of an exhaust system, for eliminating a frequency component in a certain range, and an auxiliary silencer for eliminating frequency components other than the above frequency component. In this auxiliary silencer, a plurality of closed pipes whose lengths are one fourth of the wavelengths of the frequency components to be eliminated are formed so as to protrude from a side wall of the secondary air passage. Thus, the ripple can be effectively reduced by providing a plurality of closed pipes that correspond to a plurality of frequency components contained in the ripple.
In the known apparatus as described above, since desired noise eliminating effects are obtained by providing the plural closed pipes whose lengths are one fourth of the wavelengths of the frequency components to be eliminated, the required number of closed pipes is increased with an increase in the number of frequency components to be eliminated, resulting in increased complexity and size of the apparatus.
If the ripple reducing device simply consists of a single closed pipe, the transmission loss takes its relative (or local) maximum values at frequencies that are odd multiples of 1/4 wavelength resonance mode frequency that is determined by, for example, the shape of the closed pipe, and therefore harmonics that are even multiples of the resonance mode frequency cannot be effectively reduced. Namely, the ripple reducing device consisting of a single closed pipe cannot effectively reduce the fundamental wave of the ripple and its secondary, tertiary and higher harmonics at the same time.